In consumable electrode arc welding, one of the recognized modes of operation is the short circuiting mode, wherein a power supply is connected across the consumable electrode, or welding wire, and the workpiece onto which a weld bead is to be deposited. As an arc is created, the end of the electrode melts to form a globular mass of molten metal hanging on the electrode and extending toward the workpiece. When this mass of molten material becomes large enough, it bridges the gap between the electrode and the workpiece to cause a short circuit. At that time, the voltage between the electrode and the workpiece drops drastically thereby causing the power supply to drastically increase the current through the short circuit. Such high current flow is sustained and is actually increased with time through the molten mass as the power supply inductance is overcome. Since this short circuit current continues to flow, an electric pinch necks down a portion of the molten mass adjacent the end of the welding wire. The force causing the molten welding wire to neck down is proportional to the square of the current flowing through the molten metal at the end of the welding wire. This electric pinch effect is explained by the Northrup equation: ##EQU1## I is current density, r is the distance from the center of the welding wire and R is the diameter of the neck. During the short circuit, there is a need for a relatively high current flow, which flow naturally results when the short circuit occurs. This high current flow is desirable to cause the neck portion of the molten mass to form rapidly into a very small area or neck which ultimately explodes like an electric fuse to separate the molten ball from the wire and allow it to be drawn into the weld pool by surface tension. This explosion of the neck causes spatter from the welding process. Spatter is deleterious to the overall efficiency of the welding operation and requires a substantial amount of cleaning adjacent the weld bead after the welding operation is concluded. Since the current flow through the wire or rod to the workpiece when the neck or fuse explodes is quite high, there is a tremendous amount of energy released by the neck explosion adding to the propelled distance and amount of spatter.
As can be seen, there is contradiction between the short circuit current which should be high to efficiently decrease the neck size by an electric pinch, but should be low to reduce the energy of the fuse explosion and, correspondingly, reduce the spatter and distance over which the spatter particles will be propelled.
A considerable amount of effort has been devoted to limiting spatter when the arc is reestablished by the explosion at the neck or fuse of the metal ball hanging from the welding wire and engaging the workpiece or weld pool. At first, it was suggested to reduce the diameter of the welding wire, i.e. use a 1/32 wire; however, this approach to reducing spatter caused all of the inefficiencies normally associated with using small welding wire. For instance, it was difficult to lay large amounts of weld bead and the wire sometimes stubbed or entered the weld pool without melting. As the wire diameter increased to overcome these problems, spatter was substantially increased. Faced with this dilemma, it was suggested that a high frequency power supply be used as taught in U.S. Pat. No. 4,544,826, incorporated by reference herein, wherein a high frequency inverter is turned off during a short circuiting condition or upon detection of a premonition of rearcing, i.e. blowing of the fuse. To prevent circulating currents when a high frequency power supply is turned off just before a fuse explosion, this United States Letters Patent illustrates a switch, SWD, which is opened to place a resistor in the output tank circuit of the solid state inverter for rapid attenuation of the circulating currents. This system is not applicable for all power supplies and is predicated upon a complex logic control system which actually forms the shape of the current curve from the time a short is detected to the time when the arc is reestablished after explosion of the neck or fuse. Reduction of current at the time of a short is by tuned attenuation, which phenomenon causes a time constant curve between time t.sub.1 and t.sub.2. At the detection of a neck or fuse which is about to blow, this same attenuation concept is employed. This feature is shown between the times t.sub.5 and t.sub.6 of this prior patent. The preselected wave shape, as shown in this patent, is heavily reliant upon the aforementioned attenuation of the output tank circuit of a solid state inverter which is a serious limitation especially in reducing the current flow through the neck itself at the moment of explosion. Such a preselected current shaping is applicable, if at all, to a high frequency solid state inverter power supply which can be internally turned off without substantial output inductance. With a substantial inductive reactance in the output circuit attenuation by the resistor in parallel with switch SWD would be difficult and not always guaranteed. Since direct current welding systems have output inductance this attenuation concept for lowering spatter has serious practical drawbacks.
Another patent showing a system for creating a repetition of a current cycle originally triggered by a short circuit detection is U.S. Pat. No. 4,546,234. Again, the current wave form is somewhat fixed. After a preselected time delay, current is applied across the shorted molten metal globular or ball to facilitate metal transfer. A constant current is maintained until necking is predicted, at which time the current drops rapidly to a low level and then immediately shifts up to a second high level. This system causes preselected current wave forms which are complex and generally usable, if at all, only with a high frequency solid state inverter type power supply.
As can be seen, there is a definite need for a relatively simplified system for reducing weld spatter by exerting a limited amount of actual control over weld current flow so that the current flow can assume natural operating characteristics over most of the cycle between the short and the fuse explosion. In addition, there is a substantial demand for a spatter reducing circuitry to be used with both transformer fed and solid state inverter type power supplies which do not depend upon output attenuation of low inductance circuits nor upon several distinct current level limitations.